When most of the warp was in place, I attached the collar to the neck edge.
Finally I attached the edge strands. These ones continue down the front and around the hem.
Once the weaving was done, I undid all the holding stitches. Then I could see that the edge of the collar got stretched out of shape while weaving.
I pulled it to the right shape with gathering thread. Then I worked from the back and pulled out loops of the warp threads until the collar lay where it should. Then I cut the loops and buried the ends.
Once the collar was fixed, I pressed all the edges and began to experiment with the placement of the zip.